1. Field of the Invention
The present invention relates generally to a method of controlling exposure of a process camera and an apparatus therefor, and more particularly, to a method of controlling halftone photography using sub-exposure in a process camera and an apparatus therefor.
2. Description of the Related Art
Obtaining halftone negative image of an original by using a screen such as a contact screen and a halftone screen, is called halftone photography. In halftone photography, contrast of the resultant halftone negative image should correspond to different originals having different densities. The exposure at the halftone photography should be accordingly controlled with high precision.
The halftone negative image is required for the following reasons. In letterpress printing, offset printing, screen printing and the like, continuous tones of an original having contrast such as a photograph are converted to tones represented by the dot areas of the halftone negative image because the continuous tones are difficult to obtain by a printing press or the like. The reason for it is that the printing press basically carries out printing by pressing the surface with ink applied thereto onto the paper with specific pressure, and therefore does not allow a subtle adjustment of the pressure as a person does.
Referring to FIG. 1, suppose an original having a step-like density distribution as shown in FIG. 1(b) is exposed to a film through a contact screen by a transmitted light of specific intensity shown in FIG. 1(a). Hereinafter, "density" of an original is defined as follows throughout this specification and claims attached thereto.
A density Dt of a transparent original (an original through which a light is transmitted) such as a film can be expressed by the following equation: EQU Dt=log (I.sub.0 /I.sub.t) 1)
where light intensity of an incident light on the original is represented as I.sub.0 and an intensity of the light after transmitting through the original is represented as I.sub.t.
It can be seen from the equation (1) that the higher the density Dt is, the smaller the intensity of the transmitted light is. The intensity of the light transmitted through the original having the density distribution as shown in FIG. 1(b) will be shown accordingly by FIG. 1(c). In FIG. 1(c), the light intensity is represented as a width of the arrow. The density Dt defined by equation (1) is referred to as a transmission density throughout the specification and the claims.
On the other hand, printed matters and photographs are not transparent. Therefore, for such an original (reflection original), its density Dr is defined as follows. A standard sample is illuminated with a predetermined light and the intensity Is of the reflected light for measured. The sample for comparison is illuminated with the same light and the intensity Ir of the reflected light is obtained. The density Dr will be expressed by the following equation. EQU Dr=log (Is/Ir) (2)
The density Dr defined by equation (2) is referred to as a reflection density throughout the specification and the claims.
For a contact screen, its transmission density Dt is called "screen density". The distribution of screen density of a contact screen in general is shown in FIG. 1(d).
The light of such amount as shown in FIG. 1(c) is transmitted through the contact screen having the density distribution as shown in FIG. 1(d) to reach the film. A film called a lith film is generally used for a photomechanical process. The lith film produces dots when exposed to a light (amount is referred to as an integral exposure amount) of more than a specific amount.
Therefore, when a smaller amount of light is transmitted through the original, dots are formed at only the portions where the densities of the contact screen are low. As the light amount transmitted through the original increases, the lith film produces larger dots. In this way, the halftone negative image is obtained having larger dots representing lower density (brighter) portions of the original and smaller dots representing higher density (darker) portions of the original.
A density distribution of a contact screen is peculiar to the contact screen. In addition, a density range of an original which can be reproduced through one exposure by using a contact screen having a predetermined density distribution is defined by a maximum value and a minimum value of the density of the contact screen. Note that a halftone screen, having a density distribution such that light transmitting portions and light shielding portions are alternately arranged, can provide a density distribution equivalent to the one shown in FIG. 1(d) when the halftone screen is not in contact with the photosensitive material.
Referring to FIG. 2, a difference between the maximum value and the minimum value of the screen density is referred to as "screen range" throughout the specification and the claims. A difference between the maximum value and the minimum value of an original density is referred to as "original range" throughout the specification and the claims.
In practice, original ranges of the originals to be reproduced are different from each other. To reproduce an original having an original range out of a screen range and acquire a halftone negative image in a precise tone requires a special controlling method of exposure.
One of the methods for controlling contrast of a halftone negative image corresponding to a various originals in halftone photography using a process camera is a sub-exposure method. This method generally includes a main exposure and either or both of a bump exposure and a flash exposure.
Referring to FIG. 3, the main exposure is carried out as follows. An original is placed on an original table (not shown) of a process camera. A film 37 and a contact screen 36 put on the original 35 side of the film 37 are supported by a vacuum board (not shown) or the like above the original 35. A lens 11 for forming an image of the original on the film 37 is provided between the original 35 and the film 37. The space between lens holders supporting the lens 11 and the film 37 is light-shielded from the outside by bellows 8, thereby forming a dark space. A flash lamp 14, for use in a flash exposure which will be described later, is provided on the upper surface of the lens holder. A light source 19, for reflection, is provided at the side of the bellows 8 above the original. When the original 35 is transparent, for example film, a light source 38 below the original 35 is used.
The light emitted from the light source 19 is reflected by the original 35. The reflected light forms an image of the original 35 on the film 37. The flash lamp 14 is not used. An image is formed on the film 37 through the lens 11 through the contact screen 36. The image has various sizes of dots corresponding to the tones of the contrast of the image of the original 35.
The flash exposure makes contrast low between a shadow portion and a middle tone portion of an original. Referring to FIG. 4, in the flash exposure, the film 37 is uniformly exposed through the contact screen 36 by the flash lamp 14 provided on the lens holder of the bellows 8.
A bump exposure makes contrast high between a highlight portion and a middle tone portion of an original. Referring to FIG. 5, the contact screen is removed from the film 37. The original 35 is illuminated by the light source 19 or 38 and the reflected light or the transmitted light reaches the film 37.
Referring to FIG. 6, the flash exposure has the following effects. Suppose that a latent image shown by the curve 41 of FIG. 6 is formed through the main exposure. The flash exposure increases the exposure amount as shown by the broken curve 42. As a result, a part of the latent image reaches the integral exposure amount to form dots 43a and 43b. The dot 43b is a plan view of the formed dot. Only the dots of the middle to the lowest tone grow through the flash exposure. A shadow portion has a lower contrast and the screen range is enlarged.
Referring to FIG. 7, the bump exposure has the following effects. The exposure amount of the bump exposure is about 2-10% of the main exposure amount. In the bump exposure, as an original density is increased, a smaller amount of light is applied to a film. As a result, a middle tone portion and a highlight portion are exposed to a larger extent because the corresponding portions of the original have low densities. Therefore, the dot area of latent images 45a and 45b of the middle tone to the largest dot formed by the main exposure is increased, resulting in the dots having the larger area as latent images 44a and 44b. The bump exposure makes high the contrast in the highlight portion and reduces the screen range.
The bump exposure is especially effective for reproducing precise tones of highlight regions and for eliminating dots from a white portion as in a case of an illustration drawn on a piece of white paper (a highlight process).
In the above described sub-exposure method, a main exposure and one or both of a bump exposure and a flash exposure are properly combined thereby obtaining a halftone negative image reproducing contrast of an original with fidelity.
A conventional exposure control technique for implementing a sub-exposure method was proposed by the present assignee and was disclosed in Printing Magazine (1975, No. 8, Vol. 58, pp. 43-47) and another was disclosed in Thirty-five Years of Image Techniques (published on Oct. 11, 1978 pp. 93-99) published by the present assignee.
Referring to FIG. 9, a conventional exposure control apparatus includes a halftone photographing data input unit 121 capable of inputting and setting basic data and original data and an exposure control unit 125 for controlling a lamp 19 for illuminating original and a flash lamp 14 based on the basic data and the original data.
The halftone photograph data input unit 121 includes a main exposure basic data memory 123a for storing basic data for main exposure, a bump exposure basic data memory 123b for storing basic data for bump exposure, a flash exposure basic data memory 123c for storing basic data for flash exposure and an original data input unit 122 for storing original data.
Referring to FIGS. 8 and 9, for example, the basic data for the main exposure includes a basic main exposure time T.sub.BM for reproducing a predetermined reference original in a desired dot percentage, and densities B.sub.H0 and B.sub.S0. The densities B.sub.H0 and B.sub.S0 correspond to a dot percentage A.sub.H of a highlight region of the reference original and a dot percentage A.sub.S of a shadow portion, respectively, for halftone photography using the basic main exposure time B.sub.BM. In general, 95% and 5% are considered desired values for A.sub.H and A.sub.S, respectively.
The dot percentage represents a ratio of a halftone dot area to a unit area of an image formed on a film. If a screen range of a contact screen is B.sub.DR, the equation EQU B.sub.DR =B.sub.S0 -B.sub.H0
is established.
According to the tone characteristic curve 46 of the main exposure (T.sub.BM) in FIG. 8, the reference original can be reproduced in the density range of B.sub.H0 -B.sub.S0. As indicated by the curve 48, the maximum value of the density of the reproducible reference original is increased to B.sub.S1 through the flash exposure (T.sub.BF). On the other hand, as indicated by the curve 47, the minimum value of the density of the reproduced reference original is increased to B.sub.H1.
The basic data for a bump exposure includes a basic bump exposure time T.sub.BH and a reference original density B.sub.H1. The reference original density B.sub.H1 corresponds to an obtained dot percentage A.sub.H of a highlight region when subjected to a bump exposure for the basic bump exposure time T.sub.BH after the halftone photography exposure for the basic main exposure time T.sub.BM.
The basic data for flash exposure includes a basic flash exposure time T.sub.BF and a reference original density B.sub.S1. The reference original density B.sub.S1 corresponds to an obtained dot percentage A.sub.S of a shadow region when subjected to a flash exposure for the basic flash exposure time T.sub.BF after the halftone photography exposure for the basic main exposure time T.sub.BM.
The original data includes a density D.sub.H (a highlight density) of a highlight region of an original and a density D.sub.S (a shadow density) of a shadow region at which a highlight dot percentage A.sub.H and a shadow dot percentage A.sub.S are respectively obtained as a result of the halftone photography. A difference (D.sub.S -D.sub.H) between the shadow density Ds and the highlight density D.sub.H denotes an original range D.sub.DR.
The exposure control device 125 includes an exposure operating device 127 for calculating a main exposure amount T.sub.M, a bump exposure amount T.sub.H and a flash exposure amount T.sub.F based on the basic data and the original data to control an exposure of the process camera.
The process camera includes an autofocus control circuit (not shown) and an automatic focusing device (not shown) for automatically adjusting focus based on set magnification m. The autofocus control circuit includes the autofocus operating device. The magnification m is applied from the autofocus control circuit to an exposure operating device 127. An f-number F.sub.N0 and basic data T.sub.0 for a line exposure are also accepted by the exposure operating device 127 to carry out an exposure operation.
The conventional exposure control apparatus operates as follows. The basic data and the original data are applied to the halftone data input unit 121. The exposure operating device 127 obtains the main exposure time T.sub.M, and one or both of the flash exposure time T.sub.F and the bump exposure time T.sub.H through the operations based on the equations (3)-(6) set forth below. A prescribed exposure control is carried out based on the obtained values T.sub.M, T.sub.F and T.sub.H.
The operations of the values T.sub.M, T.sub.F and T.sub.H are separately performed in the following two cases depending on a relation between the original range and the screen range.
(I) When the original range is larger than the screen range (D.sub.DR &gt;B.sub.DR), operations are as follows.
With reference to FIG. 8, the screen range is from B.sub.H0 to B.sub.S0. It is assumed that the original range D.sub.DR (=D.sub.S -D.sub.H) corresponds to the reference original density of B.sub.H0 -B.sub.S1.
In this case, there is only one way of forming a dot with a dot percentage of A.sub.H on a portion of dot image corresponding to the point D.sub.H (=B.sub.H0) of the original and a dot with a dot percentage of A.sub.S on a portion of the dot image corresponding to the point Ds (=B.sub.S1) of the original. That is, a main exposure amount is appropriately adjusted to form a dot with a dot percentage A.sub.H corresponding to the point D.sub.H (=B.sub.H0) of the original. At this time, dots corresponding to the point D.sub.S (=B.sub.S1) of the original are scarcely formed, with reference to FIG. 8. Further flash exposure results in a formation of a dot with a dot percentage A.sub.S corresponding to the point D.sub.S (=B.sub.S1) of the original. At this time, the already formed dot with the dot percentage A.sub.H remains of the same size. That is the only way to reproduce an original having a range larger than the screen range in appropriate contrast.
At this time, the values T.sub.M and T.sub.F can be obtained as described in the following: EQU T.sub.M =T.sub.BM .multidot.10.sup.A ( 3)
wherein A=D.sub.H -B.sub.H0, and EQU T.sub.F =T.sub.BF .multidot.(1-10.sup.-C)/(1-10.sup.-F) (4)
wherein C=(D.sub.S -D.sub.H)-(B.sub.S0 -B.sub.H0)=D.sub.DR -B.sub.DR, and F=B.sub.S1 -B.sub.S0.
(II) When the original range is smaller than the screen range (D.sub.DR &lt;B.sub.DR), operations are as follows.
As shown in FIG. 8, the screen range is from B.sub.H0 to B.sub.S0. It is assumed that the original range D.sub.DR (=D.sub.S -D.sub.H) corresponds to the reference original density of B.sub.H1 to B.sub.S0.
In this case, there is only one way of forming a dot with a dot percentage A.sub.H corresponding to the point D.sub.H (=B.sub.H1) of the original and a dot with a dot percentage A.sub.S corresponding to the point D.sub.S (=B.sub.S0) of the original.
A dot with a dot percentage A.sub.S is formed at the corresponding point D.sub.S (=B.sub.S0) of the original by appropriately adjusting the main exposure amount. At this time, the dot percentage of the dot formed corresponding to the point D.sub.H (=B.sub.H1) of the original is smaller than A.sub.H. Further bump exposure results in a formation of a dot with the dot percentage A.sub.H corresponding to the point D.sub.H (=B.sub.H1) of the original. At this time, the already formed dot with the dot percentage A.sub.S remains of the same size. This is the only way to reproduce the original having a range smaller than the screen range in appropriate contrast.
Herein, the values T.sub.M and T.sub.H can be obtained in a manner as described in the following: EQU T.sub.M =T.sub.BM .multidot.10.sup.B ( 5)
wherein B=D.sub.S -B.sub.S0, and EQU T.sub.H =T.sub.BH .multidot.10.sup.D .multidot.(1-10.sup.C)/(1-10.sup.-E)(6)
wherein C=(D.sub.S -D.sub.H)-(B.sub.S0 -B.sub.H0), D-D.sub.H -B.sub.H1 and E=B.sub.H1 -B.sub.H0.
In actuality, corrections corresponding to the magnification m and a change of the common f-number F.sub.N0 are added to these equations.
Theoretically the above-described conventional exposure control system has no problem, provided that the exposure and the development can be strictly controlled. However, quality of photosensitive material varies in sensitivity from one production lot to the next. Also, in an automatic development, activity of developer used therein varies to some extent. Furthermore, it is sometimes difficult to accurately measure a highlight area density or a shadow area density of the original data. Consequently, it may not be possible to obtain desired dot percentages A.sub.H and A.sub.S of the portions corresponding to the highlight region and the shadow region of the halftone negative image obtained based on the basic data and original data.
For the purpose of correction, according to the above-described conventional method, basic data is obtained whenever necessary or an appropriate exposure time is reestimated based on the exposure result to repeat halftone photographing again. The basic data, however, includes the large amount of data as described above and it requires much time to obtain basic data whenever the necessity arises. This operation requires technical knowledge of density of an original and a contact screen for use. Appropriate estimation of exposure time is made based on experience. It is not possible for an operator lacking experience to obtain good reproduction.